1. Field of the Invention
The present invention relates to a NOx gas detecting apparatus. More particularly, the present invention relates to a NOx gas detecting apparatus capable of detecting a nitrogen oxide gas (a NOx gas) contained in exhaust gas discharged from boilers, automobiles and the like, or in living environments with high sensitivity.
2. Description of Related Art
When voltage is applied between electrodes that are sintered onto each side of an oxygen ion conductive solid electrolyte (hereinafter referred to simply as a xe2x80x9csolid electrolytexe2x80x9d), such as a stabilized zirconia as a typical example, then an oxygen pumping action is produced in that oxygen is discharged from one electrode (cathode) to the other electrode (anode).
Also, when electrodes that are sintered onto each side of a solid electrolyte (hereinafter referred to as a xe2x80x9ccellxe2x80x9d) are exposed in an atmosphere including an oxygen bound gas, such as NOx, SOx, H2O, CO2 and the like, and when voltage is applied between the electrodes, the oxygen bound gas is decomposed around the cathode and the oxygen generated thereby is discharged to the anode by an oxygen pumping action of the solid electrolytes. Simultaneously, current follows from the anode to the cathode in proportion to the concentration of the oxygen bound gas decomposed.
The voltage at which this oxygen pumping action is initiated differs depending on each oxygen bound gas. In addition, even in the case of the same oxygen bound gas, the voltage at which an oxygen pumping action is initiated in association with gas decomposition differs depending on a material constructing the electrodes (especially the cathode) disposed onto the electrolyte. Further, when heated in an atmosphere of a low partial pressure of oxygen, an oxygen bound gas is decomposed generally into oxygen and other components which have been bounded to the oxygen. The partial pressure of oxygen of which decomposition rate reaches a certain value differs depending on each oxygen bound gas.
Taking advantage of an oxygen pumping action of a cell and difference in decomposition of an oxygen bound gas owing to a gas diffusion rate determining body, voltage applying conditions, electrode materials, or partial pressure conditions of oxygen, a gas detecting apparatus capable of detecting concentration of a specific oxygen bound gas contained in a measurement gas to be measured may be produced. Especially, a NOx gas detecting apparatus for detecting concentration of a NOx gas contained in a measurement gas has been put into actual use as a sensor for monitoring an amount of NOx gas contained in exhaust gas discharged from combustion equipment and combustion facilities such as automobiles and boilers.
It is required that a NOx detecting apparatus detect a NOx gas contained in exhaust gas in a minute amount independent of influence of oxygen which is a main component of the exhaust gas. To meet this end, a NOx detecting apparatus comprises a gas diffusion rate determining body, an oxygen pumping cell, and a NOx gas detecting cell. The gas diffusion rate determining body is constructed for restricting gas diffusion flowing into the oxygen pumping cell and the NOx detecting cell by providing a small pinhole or a porous body at an upstream stage of the oxygen pumping cell. The oxygen pumping cell is a cell for selectively removing nothing but oxygen from the measurement gas through the use of an oxygen pumping action of the solid electrolyte. Accordingly, for an cathode of the oxygen pumping cell, an electrode having a high activity relative to an oxygen gas but is inactive or low active relative to a NOx gas is used. As such an electrode, for example, a cermet electrode composed of a Pt-Au alloy and a ceramic component (hereinafter referred to as a xe2x80x9cPt-Au electrodexe2x80x9d) is known.
The NOx detecting cell is a cell for decomposing NOx contained in the measurement gas from which oxygen has been removed whereby a value of the current flowing between the electrodes at that time is measured. Accordingly, for a cathode of the NOx detecting cell, an electrode having a high activity relative to NOx gas is used. As such an electrode, for example, a cermet electrode composed of Pt and a ceramic component (hereinafter referred to as a xe2x80x9cPt electrodexe2x80x9d), a cermet electrode composed of a Pt-Rh alloy and a ceramic component (hereinafter referred to as a xe2x80x9cPt-Rh electrode, xe2x80x9d and the like are known). (See, for example, Japanese Unexamined Patent Publication No. HEI 11-183434 for reference.)
A NOx gas is apt to decompose when heated in a low oxygen atmosphere containing a reduction component such as an uncombusted fuel. Consequently, in order to find out about an accurate NOx concentration in exhaust gas with the NOx gas detecting apparatus, the NOx concentration needs to be measured immediately after oxygen has been removed from the measurement gas and before the measurement gas is influenced by other components. To meet this end, in the NOx detecting apparatus, the NOx detecting cell is usually disposed at a downstream stage of the oxygen pumping cell in adjacent thereto.
The NOx gas detecting apparatus constructed as described above is produced generally by printing paste containing an electrode material on a surface of a green sheet containing a solid electrolyte, laminating it onto another integrally, and then sintering it in an atmosphere at a high temperature (in the case of a zirconia-base solid electrolyte for example at 1,400xc2x0 C. or higher).
In the technical back ground described above, Pt is active for both an oxygen gas and a NOx gas, but becomes active only for oxygen by adding Au to Pt. Accordingly, a Pt-Au alloy is especially suitable as an electrode material of an oxygen pumping cell. On the other hand, Au which is contained in a Pt-Au alloy has a low temperature resistance (the melting point of 1,064xc2x0 C.), and therefore heating a Pt-Au alloy at a high temperature causes Au contained in the alloy to scatter in all directions.
Due to the above reasons, there is a problem in the case of using a Pt-Au electrode as the cathode of the oxygen pumping cell, and a Pt electrode as the cathode of the NOx detecting cell. When a green sheet constructing the oxygen pumping cell and a green sheet constructing the NOx detecting cell are laminated and sintered at a high temperature, Au contained in the cathode of the oxygen pumping cell scatters and adheres to the Pt electrode of the NOx detecting cell. This causes the NOx-reducing ability of the Pt electrode to be reduced.
Further, fuel used for automobiles or combustion facilities such as a boiler contains various impurities. Also, in exhaust gas discharged from combustion facilities, there exist components derived from additives contained in engine oil. These components also cause a problem of reducing electrode activity drastically if they adhere to the Pt electrode used in the NOx detecting cell no matter how minute the amount is.
Alternatively, a Pt-Rh electrode that contains a large amount of Rh relative to Pt (about 40 wt %) may be used as the electrode of the NOx detecting cell, because Rh has a high NOx reducing ability.
However, since Rh is strongly bounded to oxygen, if the electrode is left in an oxidation atmosphere, oxygen is adsorbed to Rh thereby forming an oxide film on the surface thereof. This causes a problem of lowering the NOx reducing ability. Here, in order to improve the NOx reducing ability, the oxide film formed on the surface of Rh needs to be removed so that Rh is used as a metal. Yet, there is a problem in that the reduction process takes a long time.
In addition, even if the reduction process is carried out, Rh again adsorbs oxygen with a lapse of time. Consequently, when voltage is applied to the NOx detecting cell, oxygen gas that has been adsorbed within the Pt-Rh electrode is discharged gradually immediately after the activation. Consequently, the NOx detecting cell appears to output as if there existed a NOx gas although there is no NOx gas, which results in a problem in that data with repeatability are not obtained. Accordingly, it is difficult to detect a NOx gas at a low concentration.
Further, the amount of oxygen gas adsorption to Rh greatly changes depending on oxygen concentration in the atmosphere and the lapse of time, and the rate of adsorption/desorption is rather slow. Due to these reasons, even if the measurement gas supplied to the NOx gas detecting cell is controlled to be constant in its oxygen concentration by using the oxygen pumping cell, it is difficult to keep the Pt-Rh electrode always at a constant state under the conditions where oxygen concentration in the atmosphere fluctuates depending on driving state as in the case of automobiles. Hence, it is difficult to faithfully monitor the state in which abrupt changes occur in the combustion state and thus in the amount NOx gas generated.
Also, this NOx detecting apparatus is provided with a gas diffusion rate determining body at an upstream stage of the oxygen pumping cell. Therefore, when a measurement gas is supplied to the NOx detecting cell and voltage applied between the electrodes is increased, a limiting current characteristic is exhibited, that is, the output current remains generally constant irrespective of applied voltage. The minimum voltage at which the output voltage becomes constant (hereinafter referred to as a xe2x80x9climiting current generating voltagexe2x80x9d) differs depending on the types of oxygen bound gas. For example, exhaust gas contains H2O and CO2 other than NOx gas, and H2O and CO2 are less decomposable than NOx so that its limiting current generating voltage is higher than that of NOx.
Also, the limiting current generating voltage relative to oxygen bound gas differs depending on the composition of the electrodes. In the case of detecting concentration of NOx in the exhaust gas using the NOx detecting cell, the cathode of the NOx detecting cell having a high limiting current generating voltage relative to NOx makes it difficult to detect NOx at a low concentration with high accuracy. This is because H2O and CO2 are decomposed along with NOx thereby to produce oxygen.
The present invention has been made in view of the above circumstances and has an object to overcome the above problems and to provide a NOx gas detecting apparatus capable of detecting a NOx gas accurately and stably over a long period of time independent of adherents to the cathode of the NOx detecting cell, such as Au, impurities contained in exhaust gas, and the like.
Another object of the present invention is to provide a NOx gas detecting apparatus which is excellent in startability, and which is capable of faithfully detecting abrupt fluctuations in the NOx concentration.
Yet, another object of the present invention is to provide a NOx gas detecting apparatus capable of accurately detecting NOx at a low concentration even if the measurement gas contains other oxygen bound gases such as H2O, CO2 and the like besides NOx.
To achieve the objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, in the present invention, a NOx gas detecting apparatus comprises an oxygen pumping cell for removing oxygen from a measurement gas, a NOx detecting cell for measuring NOx contained in the measurement gas from which oxygen has been removed. The NOx detecting cell comprises, as the cathode, an electrode composed of at least one selected from the group consisting of: a Pt-Pd alloy; a Pt-Au-Pd alloy; and a Pt-Pd-Rh alloy.
The NOx gas detecting apparatus of the present invention configured as constructed above uses an alloy including Pd as the cathode of the NOx detecting cell so that the activity relative to NOx is enhanced drastically. In addition, the NOx gas detecting apparatus allows the activity relative to NOx not to be lowered even if Au or other impurities contained in exhaust gas adhere to the electrode. Further, the Pt-Pd alloy or the Pt-Au-Pd alloy is excellent in oxidation resistance and fast in the adsorption/desorption rate as compared with the Pt-Rh alloy. Therefore, the NOx gas detecting apparatus is capable of detecting NOx accurately and stably over a long period of time. Also, the NOx gas detecting apparatus is excellent in startability and capable of faithfully detecting abrupt fluctuations in the NOx concentration, and also excellent in responsivity and repeatability. Further, through optimization of the electrode composition, the NOx gas reducing ability may be further improved as compared with the Pt electrode or the Pt-Rh electrode, thereby allowing a NOx gas to be detected in a minute amount accurately.
In the case that the NOx gas detecting apparatus comprises, as the cathode, an electrode composed of a Pt-Pd-Rh alloy, since Rh is further added to the Pt-Pd alloy, the limiting current generating voltage relative to a NOx gas is lowered, and thus a NOx gas may be discharged at a low voltage. As the result, even if the measurement gas contains oxygen bound gases (for example H2O) other than NOx, little influence is exerted on output current generated when NOx is discharged, and therefore NOx is detected at a low concentration with higher accuracy.